1. Field of the Invention
This invention relates to a cogeneration system, and particularly to a cogeneration system having an improved operation efficiency.
2. Description of the Related Art
In recent years, cogeneration systems have been developed that are equipped with an internal combustion engine-driven generator for installation in an AC power supply line between a commercial power network and an electrical load for supplying or outputting power to the load in interconnection with the power network and also for supplying hot water or the like heated by the exhaust heat (thermal output) from the engine to a thermal load.
Generally, such a cogeneration system is operated with focus on electricity (rather than heat) in which electric power is generated in response to power demand in an electrical load and thermal output generated with the power generation is utilized as much as possible, while surplus thermal output is radiated (released), or is operated with focus on heat (rather than electricity) in which electric power is generated so as to output exhaust heat in response to thermal demand in a thermal load.
Therefore, when the power demand is kept constant, but the thermal demand is increased, since an amount of generated power remains unchanged in the electricity-focused operation, it results in shortage of the thermal output. On the other hand, in the heat-focused operation, since power generation is increased with increasing thermal output, it results in surplus electricity.
For that reason, in a technique taught by Japanese Laid-Open Patent Application No. Hei 8(1996)-4586, when surplus electricity is generated, the surplus electricity is supplied to a heater installed in a water storage tank to heat the water, i.e., the surplus electricity generated by a generator is stored as thermal energy for improving energy-saving efficiency.
However, the technique mentioned in the reference is intended to merely efficiently use the surplus electricity and is not for reducing the surplus electricity itself. Specifically, since the surplus electricity or surplus thermal output lowers operation efficiency of the entire cogeneration system, it is preferable not to generate the surplus electricity or thermal output. Nevertheless, due to change in demand, the electricity-focused operation or heat-focused operation inevitably produces the surplus electricity or thermal output.